It is necessary that the above described pickup lens have a short optical length, defined as the distance from the incidence plane on the object side of the pickup lens to the image formation plane (the pickup surface of the CCD or similar). That is, in lens design, measures are necessary to reduce the ratio of the optical length to the combined focal length of the pickup lens. Hereafter, a pickup lens with short optical length and small ratio of optical length to focal length may be called a compact lens.
Taking a portable telephone set as an example, at the least the optical length must be shorter than the thickness of the telephone set main unit. On the other hand, it is convenient that the back focus, defined as the distance from the exit plane on the image side of the pickup lens to the pickup surface, be as long as possible. That is, measures are necessary in the lens design to increase as much as possible the ratio of the back focus to the focal length. This is because of the need to insert a filter, cover glass, and other components between the pickup lens and the pickup surface.
In addition to the above, pickup lenses are of course required to have various aberrations corrected sufficiently that image distortion is not noticed visually, and moreover aberrations should be corrected so as to be sufficiently small as required by the integrated density of the image pickup element (also called “pixels”). That is, the various aberrations must be satisfactorily corrected; hereafter, an image in which aberrations have been satisfactorily corrected may be called a “satisfactory image”.
As described below, pickup lenses with a three-element configuration, suitable for use in image capture equipment employing CCDs, CMOS devices and other solid-state image pickup elements, of which portable computers and videophones are representative, have been disclosed. Such lenses secure a broad angle of view, and can be made compact and lightweight.
Among these, a pickup lens has been disclosed, as a first lens with a three-element configuration, which provides satisfactory images while securing a broad angle of view (see for example Patent Document 1).
However, the shapes of the three lens elements, which are first, second, and third lenses are arranged in order from the object side, are those of a meniscus lens with convex surface on the image side and having positive refractive power as the first lens, a meniscus lens with convex surface facing the object side and having negative refractive power as the second lens, and a convex lens having positive refractive power as the third lens; and the construction has too long an optical length for the back focus length. As a result, the lens cannot be used as a compact lens.
As second through fourth three-element lenses, pickup lenses with a short focal length have been disclosed in which various aberrations have been satisfactorily corrected while securing a broad angle of view (see for example Patent Document 2, Patent Document 3, Patent Document 4).
Similarly to the above-described pickup lens, these pickup lenses also comprise first, second, and third lenses arranged from the object side, with the first lens having positive refractive power, the second lens having negative refractive power, and the third lens having positive refractive power. The combined focal length of the pickup lens is set to be short; but the back focus is long relative to the combined focal length, and the optical length is also too long. In addition, the lenses use a glass material, and so costs are high.
As a fifth three-element lens, a pickup lens has been disclosed which is made compact by employing an aspherical lens and by appropriately setting the power distribution and surface shape (see for example Patent Document 5).
However, in this pickup lens, comprising three lenses which are first, second, and third lenses arranged in order from the object side, the first lens has negative refractive power, the second lens has positive refractive power, and the third lens has negative refractive power; as a result, the pickup lens has a long optical length relative to the combined focal length. In addition, the lenses use a glass material, and so costs are high.
A sixth three-element lens has been disclosed which is a plastic lens with a set of meniscus-shape lenses having concave surfaces directed toward each other, and having at least one plastic lens with an aspherical surface, employing three lenses in the entire lens system to achieve compact size and low cost, and enabling easy suppression of focus movement accompanying temperature changes (see for example Patent Document 6).
However, in this pickup lens, in which first, second, and third lenses are arranged in order from the object side, the first lens has weak refractive power, the second lens has weak refractive power, and the third lens has positive refractive power; consequently the third lens alone is not able to completely compensate for the refractive powers of the first and second lenses, and as a result the back focus is long relative to the combined focal length, and the optical length is also long. However, because the third lens is a glass lens, cost reduction is also inadequate.
A seventh three-element lens has been disclosed in which the lens system is divided into a front group and a rear group, in a telescopic-type lens configuration in which the front group has positive refractive power and the rear group has negative refractive power; the optical length is short, and the lens system is inexpensive (see for example Patent Document 7).
However, in this lens system, in which first, second, and third lenses are arranged in order from the object side, the first lens has negative refractive power, the second lens has positive refractive power, and the third lens has negative refractive power, with a broad gap between the second lens and the third lens. For this reason the optical length is long relative to the combined focal length, and moreover there is the problem that the third lens has large diameter, making the pickup lens unsuitable for use in installation in devices for image input to portable telephone sets and personal computers or in digital cameras, CCD cameras for monitoring, inspection devices, or similar.
An eighth three-element pickup lens, having two positive lenses on the object side as well as a concave negative lens on the image side, both surfaces of which are aspherical, and the negative power of which gradually weakens in moving from the lens center toward the periphery until at the peripheral portion the power is positive, has been disclosed (see for example Patent Document 8).
However, a characteristic of this lens system is that the lens equivalent to the third lens L3 has negative power which gradually weakens in moving from the lens center toward the periphery, such that there exists a position at which the power changes to positive power, at a distance from the lens center in the range from 0.7 times to 1.0 times the effective aperture. In lenses disclosed as embodiments, the positions at which the power changes to positive are at distances from the lens center of 0.96 and 0.97 times the effective apertures, and so set nearly in the peripheral portion of the lens.
If the position at which the power changes to positive is set in the peripheral portion of the lens, then the angle of incidence on the image pickup element of light incident on the vicinity of intersection of the lens optical axis with the pickup surface and on the peripheral portion is nearly perpendicular, while at a position intermediate between the point of intersection of the lens optical axis and the pickup surface and the lens peripheral portion, the angle of incidence on the image pickup element deviates greatly from perpendicular. Hence the angle of incidence of light at positions intermediate from the lens peripheral portion, which are of importance to the image, deviates greatly from the perpendicular, so that light is incident on the image pickup element from an angle oblique to the image pickup element and the amount of reflection at the pickup surface increases, the amount of optical energy reaching the photoelectric conversion face of the image pickup element is reduced, and consequently there is the problem that the image in this portion becomes dark.
As a ninth three-element configuration, a pickup lens has been disclosed in which are arranged, in order from the object side, an aperture diaphragm, a first lens with a biconvex shape having positive refractive power, a second lens with a concave surface directed toward the object side and having negative refractive power, and a third lens with a meniscus shape, with the convex surface directed toward the object side (see for example Patent Document 9).
This lens system is designed such that a satisfactory image can be obtained when the aperture diaphragm is positioned on the object side of the first lens. By positioning the aperture diaphragm on the object side of the first lens, the position of the entrance pupil can be brought close to the object. By this means, there is the characteristic that the principal ray can be made incident on the pickup surface at a nearly perpendicular angle. If the principal ray is made incident on the pickup surface at an oblique angle, shading occurs, in which the amount of light incident on pixels (the image pickup element) positioned in the pickup surface is reduced, and so a problem arises in which the image is darker in the peripheral portions of the picture area.
This problem arises from the fact that when a ray is incident on the image pickup element at an oblique angle to the image pickup element, the amount of light reflected at the surface of the image pickup element increases, and the amount of optical energy reaching the photoelectric conversion face of the image pickup element is reduced. That is, by positioning the aperture diaphragm on the object side of the first lens, a pickup lens design is possible in which shading does not readily occur.
When, in a lens system designed based on such design principles, a diaphragm is further placed between the first lens and the second lens for the purpose of preventing flare, which is a phenomenon in which contrast in the image is decreased, or smear, which is a phenomenon of blurring of the image, the following may occur. Among the principal ray passing through the aperture diaphragm, the principal ray incident at a large angle of incidence to the optical axis of the pickup lens is blocked by this diaphragm. As a result, although stray light which may cause flare or smear and detract from image quality may be blocked, a portion of the principal ray is blocked, as described above, and in some cases the amount of light at the periphery of the image may be decreased, giving rise to the problem of darkness at the peripheral portions of the image.
This lens system has, as the lens equivalent to the third lens, a meniscus lens having positive refractive power; as a characteristic resulting from this, the back focus is relatively short compared with the optical length. Hence, if the back focus is made long in order to insert a filter, cover glass and other components between the pickup lens and the pickup surface, the optical length also becomes long, and there is the problem that the lens system itself becomes too large.
As a tenth three-element lens, a pickup lens has been disclosed in which are arranged, in order from the object side, a first lens with convex shape on the object side and having positive refractive power; a diaphragm; a second lens of a plastic material, at least one surface of which has an aspherical shape, with concave surface on the object side and having positive or negative refractive power; and, a third lens, both surfaces of which have an aspherical shape, with a convex surface on the object side and having positive refractive power (see for example Patent Document 10).
This tenth three-element lens is designed so as to obtain satisfactory images on the assumption that a diaphragm is set between the first and second lenses, and that this diaphragm functions as an aperture diaphragm. Hence when a shutter or similar is placed on the object side of the first lens, the incidence aperture to this lens is narrowed by the shutter or similar. That is, the shutter or similar effectively functions as a diaphragm, so that a portion of the principal ray incident on the diaphragm is blocked. The principal ray incident on the lens at a large angle with the optical axis is a ray which forms the image in the peripheral portion; this ray is blocked by the shutter or similar placed on the object side of the first lens, and so there is the possibility of a problem in which the peripheral portion of the image becomes dark.
In addition, in this lens-system also, similarly to the above-described ninth three-element lens, the lens equivalent to the third lens L3 is a meniscus lens having positive refractive power. Hence in this lens system also, similarly to the ninth three-element lens, if the back focus is made long then the optical length becomes long, and there is the problem that the lens system itself becomes too large.
As an eleventh three-element lens, a pickup lens has been disclosed in which are arranged, in order from the object side, a first lens of a glass material, with a convex surface shape on the object side and having positive refractive power; a second lens with a meniscus shape, formed of a plastic material, at least one surface of which is an aspherical shape, with concave surface on the object side and having positive refractive power; and a third lens, formed from a plastic material, both surfaces of which have an aspherical shape, with convex surface on the object side, and having positive or negative refractive power (see for example Patent Document 11).
The basic configuration of the eleventh three-element lens is the same as that of the tenth three-element lens, and so there are problems similar to those described above for the tenth three-element lens.
As a twelfth three-element lens, a pickup lens has been disclosed in which are placed, in order from the object side, a first lens having at least one surface of which is aspherical in shape, with both surfaces having convex shapes, and having positive refractive power; a diaphragm; a second lens with a meniscus shape, at least one surface of which is aspherical in shape, which has a convex surface on the object side and positive refractive power; and a third lens, both surfaces of which are aspherical in shape, having positive or negative refractive power, formed of a plastic material, and with a convex surface shape on the object side (see for example Patent Document 12).
The basic configuration of the twelfth three-element lens is similar to that of the above-described tenth and eleventh three-lens configuration lenses. Hence there are problems similar to the above-described problems of the tenth and eleventh three-element lenses.
As a thirteenth three-element lens, a pickup lens has been disclosed in which there are, placed in order from the object side, a first lens with a convex surface on the object side and with mainly positive refractive power; a second lens, in a meniscus shape, with convex surface on the image side and having negative refractive power; and a third lens, with a convex surface on the object side, and having positive refractive power. Pickup lenses have been disclosed which have a diaphragm placed on the object side of the first lens, and which have a diaphragm placed between the first lens and the second lens (see for example Patent Document 13).
That is, a pickup lens designed on the assumption that satisfactory images are obtained by causing a diaphragm placed on the object side of the first lens to function as an aperture diaphragm, and a pickup lens designed on the assumption that satisfactory images are obtained by causing a diaphragm placed between the first and second lenses to function as an aperture diaphragm, have been disclosed.
As explained above, when a diaphragm is further placed between the first lens and the second lens in a pickup lens designed so as to obtain satisfactory images on the assumption that a diaphragm placed on the object side of the first lens is made to function as an aperture diaphragm, the principal ray incident at a large incidence angle with the optical axis of the pickup lens among the principal ray passing through the aperture diaphragm is blocked by the additionally placed diaphragm. Similarly, when a diaphragm is placed on the object side of the first lens of a pickup lens designed so as to obtain satisfactory images on the assumption than a diaphragm placed between the first lens and the second lens is made to function as an aperture diaphragm, the principal ray incident at a large incidence angle with the optical axis of the pickup lens among the principal ray passing through the aperture diaphragm is blocked by the additionally placed diaphragm.
As a result, as described above, while stray light which may cause flare, smear or similar, detracting from image quality, can be eliminated, a portion of the principal ray is blocked as described above, and in some cases the amount of light at the periphery of the image may be decreased, giving rise to the problem of darkness at the peripheral portions of the image.
In Patent Document 13, an embodiment is disclosed in which a pickup lens in which an aperture diaphragm is placed on the object side of the first lens, and a pickup lens in which an aperture diaphragm is placed between the first lens and the second lens, are designed mutually independently. That is, the shapes and positions of the first through third lenses are designed so as to obtain a satisfactory image in each case according to the position in which the aperture diaphragm is placed. Hence there is no disclosure of a pickup lens in which a diaphragm is placed on the object side of the first lens, and in addition an aperture diaphragm is placed between the first lens and the second lens. That is, there is no disclosure of a pickup lens in which, in addition to an aperture diaphragm which determines the entrance pupil position, a diaphragm is simultaneously provided for the purpose of preventing flare or smear in order to further improve lens performance.
In the thirteenth three-element lens, similarly to the above-described ninth three-element lens, the lens equivalent to the third lens L3 is a meniscus lens having positive refractive power. Hence in this lens system also, similarly to the ninth three-element lens, if the back focus is made long the optical length will also be long, and so there is the problem that the lens system itself will be too large.
As a fourteenth three-element lens, a pickup lens has been disclosed in which are placed, in order from the object side, a first lens with a convex surface on the object side, having positive refractive power; an aperture diaphragm; a second lens with a meniscus shape, with the convex surface on the image side, having positive refractive index; and a third lens with a concave surface on the image side, having negative refractive power (see for example Patent Document 14).
In this pickup lens, the value of the ratio f1/f of the focal length f1 of the first lens to the focal length of the entire pickup lens system is set so as to satisfy 0.8<f1/f<2.0. Hence the refractive power of the first lens is weak, and the optical length must be made long. Consequently the lens cannot be made compact. Further, a lens having positive refractive power is adopted as the second lens, and the radius of curvature of the surface of this second lens on the image side (the convex surface directed toward the image) must be made small. As a result the lens surface curvature is large, and so die machining becomes difficult.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-075006
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-149548
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-221659
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2002-244030
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2003-149545
Patent Document 6: Japanese Unexamined Patent Application Publication No. 10-301022
Patent Document 7: Japanese Unexamined Patent Application Publication No. 10-301021
Patent Document 8: Japanese Unexamined Patent Application Publication No. 2003-322792
Patent Document 9: Japanese Unexamined Patent Application Publication No. 2004-4566
Patent Document 10: Japanese Unexamined Patent Application Publication No. 2004-302058
Patent Document 11: Japanese Unexamined Patent Application Publication No. 2004-302059
Patent Document 12: Japanese Unexamined Patent Application Publication No. 2004-302060
Patent Document 13: Japanese Unexamined Patent Application Publication No. 2005-4045
Patent Document 14: Japanese Unexamined Patent Application Publication No. 2005-242286